Internal combustion engines may use variable cam timing (VCT) to improve fuel economy and emissions performance of a vehicle. The VCT device may include a vane type cam phaser that is controlled by an electromechanically actuated spool valve. The spool valve may direct flow of a hydraulic fluid, such as oil, from one side of the vane to the other, such as from a retard side to an advance side. The VCT device may include more than one oil circuit connecting one side of the vane to the other through which the flow of a hydraulic fluid may be directed. The phaser may be oil pressure actuated, wherein the actuation of the phaser is dependent on oil pressure in the circuit. Alternatively, the phaser may be cam torque actuated wherein the actuation of the phaser is dependent on torque generated during cam actuation.
One example of a cam torque actuated VCT phaser is shown by Smith et al. in U.S. Pat. No. 8,356,583. Therein, the VCT device is configured with a hydraulically activated locking pin in an intermediate position (herein also referred to as a mid-lock position). Conventional VCT devices may include a locking pin at one end of the range of the phaser. The VCT device of Smith also utilizes two independent oil circuits, herein referred to as the phasing circuit and the detent circuit. In the mid-lock VCT phaser of Smith, a piloted valve is included in the phaser's rotor assembly and is moveable from a first position to a second position. When the piloted valve is in the first position, hydraulic fluid is blocked from flowing through the piloted valve. When the piloted valve is in the second position, hydraulic fluid is allowed to flow between a detent line from the advance chamber and a detent line from the retard chamber through the piloted valve and a common line, such that the rotor assembly is moved to and held in the intermediate phase angle position relative to the housing assembly. Detent lines communicating with the advance chamber or retard chamber are blocked when the VCT phaser is at or near the intermediate position. The spool valve has three regions of operation, namely Detent (or Auto-Lock), Retard, and Advance in the specified order. The auto-lock region may hereupon be referred to as the detent region. Specifically, when the spool valve is commanded to the retard or advance regions, the piloted valve is in the first position, and fluid is blocked from flowing through the detent circuit lines. Additionally, fluid may flow from one side of the vane to the other via the phasing circuit lines. When the spool valve is commanded to the detent region, the piloted valve is in the second position, and fluid is free to flow from the advanced or retarded chamber, through the detent lines and the piloted valve, and into the opposite chamber through a common fluid line. Additionally, fluid is blocked from flowing through the phasing circuit lines.
However, the inventors herein have identified potential issues with such a VCT system. In the case of a cam torque actuated (CTA) VCT, the spool valve has three regions of operation, namely Auto-lock, Retard, and Advance, in the specified order. If the spool valve is commanded from a low retard region or the advance region to the auto-lock region, it must physically travel through a high retard region. In the instance that a retarded cam torsion is experienced while the spool valve is travelling through the high retard region, the cam phaser may change its position by several degrees in the retarded direction immediately before reaching the detent region and auto-locking. This may increase the time required by the detent circuit to hydraulically adjust the cam phaser position to the neutral position, particularly if the cam phaser was already positioned at the mid-lock position in anticipation of an auto-lock command. Additionally, this may create delays in subsequent engine commands that require the cam phaser to be held in a position with a locking pin engaged.
In one example, the above issue may be at least partly addressed by a method for an engine, comprising moving a spool valve of a cam torque-actuated variable cam timing phaser to a detent region in between torsional pulses of a camshaft. In this way, the effect of retarded cam torsions on cam phaser position movement is reduced.
As an example, a controller may map cam torsion events as a function of time and crankshaft position. During conditions when a spool valve is to be moved to the auto-lock region from the advance or retard regions, the spool valve may be moved through the high retard region based on the timing of the cam torsion events as well as delays associated with the electromechanical actuation of the spool valve. In particular, the spool valve may be commanded to travel through the high retard region on its way to the auto-lock region between retarded cam torsion events. In addition, spool valve commands that would move the spool valve during retarded cam torsion events may be disabled. Thus, inadvertent actuation of the cam phaser may be avoided by circumventing actuating pulses.
In this way, a cam phaser position may be adjusted with higher reliability and accuracy. In particular, by moving the spool valve based on a timing of retarded cam torsion events, unwanted position adjustments from the retard torsions may be reduced. This allows an engine controller to be able to first command the cam phaser to the mid-lock position without activating the locking pin and time the movement of the spool valve to the detent region where the locking pin is engaged in such a way that the cam phaser remains in the mid-lock position during the command. By reducing the occurrence of unwanted position adjustments arising from retarded cam torsion events, the time associated with engaging a locking pin of a VCT phaser may be made more consistent.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.